Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer an important improvement over lead-acid batteries. Typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal. The useful lifetime of flowing electrolyte batteries is, on the other hand, not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
Referring to FIG. 1, a flow diagram illustrates a basic zinc-bromine flowing electrolyte battery 100, as known according to the prior art. The zinc-bromine battery 100 includes a negative electrolyte circulation path 105 and an independent positive electrolyte circulation path 110. The negative electrolyte circulation path 105 contains zinc ions as an active chemical, and the positive electrolyte circulation path 110 contains bromine ions as an active chemical. The zinc-bromine battery 100 also comprises a negative electrolyte pump 115, a positive electrolyte pump 120, a negative zinc electrolyte (anolyte) tank 125, and a positive bromine electrolyte (catholyte) tank 130.
To obtain high voltage, the zinc-bromine battery 100 further comprises a stack of cells 135 connected in a bipolar arrangement, the stack of cells 135 producing a total voltage higher than that of the individual cells.
For example, a cell 135 comprises half cells 140, 145 including a bipolar electrode plate 155 and a micro porous separator plate 165. The zinc-bromine battery 100 then has a positive polarity end at a collector electrode plate 160, and a negative polarity end at another collector electrode plate 150.
A chemical reaction in a positive half cell, such as the half cell 145, during charging can be described according to the following equation:2Br−→Br2+2e−  Eq. 1
Bromine is thus formed in half cells in hydraulic communication with the positive electrolyte circulation path 110 and is then stored in the positive bromine electrolyte tank 130. A chemical reaction in a negative half cell, such as the half cell 140, during charging can be described according to the following equation:Zn2++2e−→Zn  Eq. 2
A metallic zinc layer 170 is thus formed on the collector electrode plate 150 in contact with the negative electrolyte circulation path 105.
Chemical reactions in the half cells 140, 145 during discharging are then the reverse of Eq. 1 and Eq. 2.
A problem with the basic zinc-bromine flowing electrolyte battery 100 of the prior art is that it has not generally been possible to strip the battery 100 of its metallic zinc layers 170 sufficiently quickly and safely, and without damaging the battery 100.
Stripping the flowing electrolyte battery 100 can be important, both as a periodic maintenance step to improve the efficiency of the battery 100, and as part of an emergency process of neutralising the stored energy in the battery 100.
Stripping the zinc-bromine flowing electrolyte battery 100 of the prior art using electrical current is both costly, as it generally requires an external maintenance bus, direct current (DC)-DC converter, or stripper circuit, and is prone to damaging the battery 100, and in particular the electrodes of the battery 100.
Furthermore, electrically stripping the zinc-bromine flowing electrolyte battery 100 quickly from full charge is generally unsafe as excessive heat can be generated.
A further problem with the prior art zinc-bromine flowing electrolyte battery 100 is that uneven zinc deposits and dendrites can both reduce efficiency of the battery 100, and even cause damage to cells of the battery 100.
Yet a further problem with the zinc-bromine flowing electrolyte battery 100 is that overcharging, for example due to a fault in an electrical charging system, can severely damage the battery 100.
There is therefore a need to overcome or alleviate many of the above discussed problems associated with flowing electrolyte batteries of the prior art.